Christian Madu, Ph.D.Collin College

Lecture Presentation

Chapter 3-1

Molecules, Compounds, and

Chemical Equations

How Many Different Substances Exist?

• Elements combine with each other to form compounds.

• The great diversity of substances that we find in nature is a direct result of the ability of elements to form compounds.

Hydrogen, Oxygen, and Water• The dramatic difference between the

elements hydrogen and oxygen and the compound water is typical of the differences between elements and the compounds that they form.

• When two or more elements combine to form a compound, an entirely new substance results.

Hydrogen, Oxygen, and Water

Definite Proportion

• A hydrogen–oxygen mixture can have any proportions of hydrogen and oxygen gas.

• Water, by contrast, is composed of water molecules that always contain two hydrogen atoms to every one oxygen atom.

• Water has a definite proportion of hydrogen to oxygen.

Definite Proportion

Chemical Bonds

• Compounds are composed of atoms held together by chemical bonds.

• Chemical bonds result from the attractions between the charged particles (the electrons and protons) that compose atoms.

• Chemical bonds are classified into two types: – Ionic– Covalent

Ionic Bonds

• Ionic bonds—which occur between metals and nonmetals—involve the transfer of electrons from one atom to another.

• When a metal interacts with a nonmetal, it can transfer one or more of its electrons to the nonmetal.

– The metal atom then becomes a cation. – The nonmetal atom becomes an anion.

Ionic Bonds

• These oppositely charged ions attract one another by electrostatic forces and form an ionic bond.

• The result is an ionic compound, which in the solid phase is composed of a lattice—a regular three-dimensional array—of alternating cations and anions.

Ionic Bonds

Covalent Bonds• Covalent bonds—which occur between two or

more nonmetals—involve the sharing of electrons between two atoms.

• When a nonmetal bonds with another nonmetal, neither atom transfers its electron to the other. Instead the bonding atoms share some of their electrons.

• The covalently bound atoms compose a molecule. – Hence, we call covalently bonded compounds

molecular compounds.

Representing Compounds: Chemical Formulas and Molecular Models

• A compound is represented with its chemical formula.

• Chemical formula indicates the elements present in the compound and the relative number of atoms or ions of each.– Water is represented as H2O.

– Carbon dioxide is represented as CO2.– Sodium Chloride is represented as NaCl.– Carbon tetrachloride is represented as CCl4.

Types of Chemical Formulas

• Chemical formulas can generally be categorized into three different types:

• Empirical formula

• Molecular formula

• Structural formula

Types of Chemical Formulas• An empirical formula gives the relative number

of atoms of each element in a compound.• A molecular formula gives the actual number

of atoms of each element in a molecule of a compound.

(a) For C4H8, the greatest common factor is 4. The empirical formula is therefore CH2.

(b) For B2H6, the greatest common factor is 2. The empirical formula is therefore BH3.

(c) For CCl4, the only common factor is 1, so the empirical formula and the molecular formula are identical.

Types of Chemical Formulas

• A structural formula uses lines to represent covalent bonds and shows how atoms in a molecule are connected or bonded to each other. The structural formula for H2O2 is shown below:

Types of Chemical Formulas

• The type of formula we use depends on how much we know about the compound and how much we want to communicate.

• A structural formula communicates the most information,

• while an empirical formula communicates the least.

• A molecular model is a more accurate and complete way to specify a compound.

• A ball-and-stick molecular model represents atoms as balls and chemical bonds as sticks; how the two connect reflects a molecule’s shape.

• The balls are typically color-coded to specific elements.

Molecular Models

Molecular Models• In a space-filling molecular model, atoms fill

the space between each other to more closely represent our best estimates for how a molecule might appear if scaled to visible size.

Ways of Representing a Compound

An Atomic-Level View of Elements and Compounds

• Elements may be either atomic or molecular. Compounds may be either molecular or ionic.

View of Elements and Compounds • Atomic elements exist in nature with single

atoms as their basic units. Most elements fall into this category.

• Examples are Na, Ne, C, K, Mg, etc.

• Molecular elements do not normally exist in nature with single atoms as their basic units; instead, they exist as molecules—two or more atoms of the element bonded together.

• There only seven diatomic elements and they are H2, N2, O2, F2, Cl2, Br2, and I2.

• Also, P4 and S8 are polyatomic elements.

Molecular Elements

Molecular Compounds• Molecular compounds are usually composed

of two or more covalently bonded nonmetals.

• The basic units of molecular compounds are molecules composed of the constituent atoms.

• Water is composed of H2O molecules.

• Dry ice is composed of CO2 molecules. • Propane (often used as a fuel for grills) is composed of

C3H8 molecules.

Ionic Compounds• Ionic compounds are composed of cations

(usually a metal) and anions (usually oneor more nonmetals) bound together byionic bonds.

• The basic unit of an ionic compound is the formula unit, the smallest, electrically neutral collection of ions.

• The ionic compound table salt, with the formula unit NaCl, is composed of Na+ and Cl– ions in a one-to-one ratio.

Molecular and Ionic Compounds

Polyatomic Ion

• Many common ionic compounds contain ions that are themselves composed of a group of covalently bonded atoms with an overall charge.

• This group of charged species is called polyatomic ions.– NaNO3 contains Na+ and NO3

–.

– CaCO3 contains Ca2+ and CO32–.

– KClO Contains K+ and ClO–.

Ionic Compounds: Formulas and Names• Summarizing Ionic Compound Formulas:

– Ionic compounds always contain positive and negative ions.

– In a chemical formula, the sum of the charges of the positive ions (cations) must equal the sum of the charges of the negative ions (anions).

– The formula of an ionic compound reflects the smallest whole-number ratio of ions.

Ionic Compounds: Formulas and Names• The charges of the representative elements can

be predicted from their group numbers.• The representative elements forms only one

type of charge.

• Transition metals tend to form multiple types of charges.

• Hence, their charge cannot be predicted as in the case of most representative elements.

Naming Ionic Compounds

• Ionic compounds are usually composed of metals and nonmetals.

• Anytime you see a metal and one or more nonmetals together in a chemical formula, assume that you have an ionic compound.

• NaBr, Al2(CO3)3, CaHPO4, and MgSO4 are some examples of ionic compounds.